Spiral antennas are a type of radio frequency antenna, used for microwave applications. Spiral antennas belong to the class of “frequency independent” antennas; these antennas have a very large bandwidth, that is, they operate over a wide frequency range. For example, their bandwidth can be as high as 30:1, which means that if the lower frequency is 1 GHz, the antenna could still be in band at 30 GHz, and every frequency in between.
The polarization, radiation pattern and impedance of spiral antennas remain unchanged over large bandwidth. They are circularly polarized with low gain. An array of spiral antennas can be used to increase the gain. Often, the spiral is cavity-backed; a cavity of air or non-conductive material or vacuum is surrounded by conductive walls, and the cavity changes the antenna pattern to a unidirectional shape.
One shortcoming of spiral antennas is that their diameter is typically ⅓rd of a wavelength at the lowest operating frequency. This means that they cannot be arrayed close enough together at higher frequencies for adequate spatial sampling of the incoming wave.